Triple bow siege crossbow

Arqai · Post by **Arqai** » Thu Jan 24, 2013 10:44 pm

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I have recently become fascinated by the idea of building a triple bow siege crossbow. Mine won't be full scale, but I'd like to be reasonably historically accurate in design. Any historical reference would be greatly appreciated, particularly regarding the bows and how they were constructed.

Thanks!
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Bede · Post by **Bede** » Wed Jan 30, 2013 1:26 am

Do you mean the Chinese style triple bowed ballista? The best technical study is McEwen, E., "The Bow of the Ox", Journal of the Society of Archer-Antiquaries, 1985, Vol. 28, pp. 15-21. He details a reconstruction he made with straight self bows that had an extraordinary draw length and was very efficient. It details the different strengths that the bows must have to draw evenly (i.e the forward bow must have half the draw weight of either of the two rear bows). He speculated on the potential performance of a similar set up with composite bows and believed it might have equalled the historical accounts.

Arqai · Post by **Arqai** » Wed Jan 30, 2013 4:12 am

Yes, that is the engine I'm interested in. Does this article mention anything regarding the bow material and dimensions? If so, how did he decide what materials to use? It looks like I need to get my hands on a copy of the article. I just have too many questions.

Thanks for the info!

Bede · Post by **Bede** » Mon Feb 04, 2013 7:59 am

McEwen believed the original bows were composite. He mentions that multiple arrow crossbows of 1870 lb were described in a work by Li Chuan in 759 CE so there was no problem building powerful bows.

I think I published a string path for these bows on ATARN some time ago. i will see if I can find it again.

Arqai · Post by **Arqai** » Mon Feb 04, 2013 1:01 pm

A friend gave me a copy of the McEwan article. If we number the bows from the "muzzle" end of the tiller as 1, 2, 3, McEwen strung his bows from the left side 1 to 3 then to 2, across the tiller to the right side of 2, back down to 3 then back up to 1. He had pulleys on bows 2 and 3 in order to get the string to behave. A lot of the artwork from the period just shows the string looped over the bow, but I don't see how you could brace any of the bows that way and McEwen found it to be completely impractical.

I've also seen a reproduction engine where the front two bows were tied together, 1 to 2. The "active" string then was fixed to bow 3, up to 2, across the tiller to the other side of 2, then back down to 3. This would mean that the imbalance of the bows would require that 3 be stronger than 1 and 2. It would also mean only 1 pair of pulleys. This is the configuration I'm planning to try.

Bede · Post by **Bede** » Mon Mar 11, 2013 2:03 am

The artists were not technical draughtsmen so be careful of putting too much reliance on their accuracy. If they left out the pulleys and the release mechanism, they probably made other mistakes.

I had made a drawing of the McEwen solution but it may be archived somewhere. I felt that the pulleys could have been supported by loops so that they did not twist the tips of the limbs, but if the limb shapes were stable, they could store energy in torsion the way Alan Case's multi-bow flight bow did.

Arqai · Post by **Arqai** » Wed Mar 13, 2013 12:56 pm

Agreed. I intend to have my "pulleys" be tied on with cordage, allowing them to "float" and avoiding limb twist. My first "pulleys" will be just simple metal rings. Friction may be an issue, but I'm hoping to limit that as well. Also, as I will have only one pulley set instead of two, I expect that the configuration will be practical enough for my purposes.

Gazzanoo · Post by **Gazzanoo** » Mon Apr 15, 2013 10:31 am

Found this site whilst browsing hope it helps: http://www.chinahistoryforum.com/index. ... -crossbow/

Bede · Post by **Bede** » Tue Apr 16, 2013 8:14 am

Here is the diagram I made following McEwen's article to show the path of the string. McEwen discovered empirically that the front bow had to half the weight of each of the other two.

Arqai · Post by **Arqai** » Tue Jun 04, 2013 3:42 am

Here's a picture of my engine at it's first trials. It seems to be under powered. My prods are stacked bamboo and will require some further attention, adding leaves either of more bamboo or of a hardwood like ash or hickory. The string arrangement worked quite well though, even with an improvised trigger.

[img]http://wiki.antir.sca.org/images/6/68/WGatJF.JPG[/img]

Bede · Post by **Bede** » Tue Jun 04, 2013 7:07 am

It is probably worthwhile angling the limbs of the second bow upwards to allow the string to clear the arrow channel. It would also reduce interference between the cables and the tips.

Arqai · Post by **Arqai** » Tue Jun 04, 2013 1:37 pm

[quote="Bede"]It is probably worthwhile angling the limbs of the second bow upwards to allow the string to clear the arrow channel. It would also reduce interference between the cables and the tips.[/quote]

It may not be visible in the picture but the second bow is tilted 15 degrees for that very purpose.

There is much work that can be done to improve performance. This first trial was very rushed in order to have something to demonstrate for an event. I'll have more detail as the project progresses.

I would be remiss if I didn't extend my thanks to this community for their input into this project. So, THANKS! Your resources and interest have contributed greatly to my learning in this project.

HeroSK · Post by **HeroSK** » Fri Jun 13, 2014 7:47 am

Hi all, could someone please inform where can I get McEwen's article? Thanks in advance.

KenH · Post by **KenH** » Fri Jun 13, 2014 11:16 am

See Bede's first post at the top of this discussion: "The best technical study is McEwen, E., "The Bow of the Ox", Journal of the Society of Archer-Antiquaries, 1985, Vol. 28, pp. 15-21." If you're a member of the Society, I believe you can get the article online.

HeroSK · Post by **HeroSK** » Sat Jun 28, 2014 8:01 am

Thank you KenH, unfortunately article is beyond my reach. While searching I found below thread in another forum. I has some useful information if anyone is interested. Also, I beg your forgiveness Bede about posting your review and image that you provided without your permission.

http://www.myarmoury.com/talk/viewtopic.php?t=15759

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